Fuel efficiency of an internal combustion engine may be improved by deactivating selected cylinders of the engine during low torque demand conditions. The engine can produce equivalent torque and conserve fuel with fewer active cylinders by increasing the cylinder air charge of active cylinders, at least during some conditions. The increased cylinder air charge can increase the thermal efficiency of the cylinder resulting in less fuel consumed for equivalent torque production. Further, a higher intake manifold pressure is often necessary to increase the cylinder air charge. By raising the intake manifold pressure cylinder pumping losses may be reduced. Consequently, for some conditions, it is possible to meet engine torque requirements with fewer cylinders and less fuel.
One internal combustion engine capable of deactivating a group of cylinders is described in U.S. Patent application No. 2005/0034701. This method provides for deactivating separate groups of cylinders that are in communication with individual turbo chargers. Further, the cylinders of one group have a different compression ratio than those of the other group. Apparently, the cylinder groups are activated and deactivated based on engine load requirements in an effort to reduce fuel consumption and emissions.
The above-mentioned internal combustion engine can also have several disadvantages. Specifically, the method does not consider that air flow through a turbo charge can be affected by activating or deactivating cylinders of an engine. For example, when an engine is operated using a first group of cylinders and then activates a second group of cylinders, the activation of the second group of cylinders can change the air flow to the turbo charger that is in communication with the first group of cylinders which can cause a change in engine torque that may be objectionable to a driver. Without a way to mitigate torque disturbances the vehicle drivability may degrade. Furthermore, the air-fuel ratios of engine cylinder groups may deviate during cylinder activation or deactivation sequences because the flow rate to a turbo charger may change. This may lead to higher engine emissions or increased exhaust after treatment costs.
The inventor herein has recognized the above-mentioned disadvantages and has developed a method to control engine valves during stopping and starting that offers substantial improvements.